In the fabrication of semiconductor devices there is an on-going need to reduce packaging costs and improve the electrical and thermal performance of the packages. Package sizing is also important, especially the profile or height of the package, when mounted to a printed wiring board or printed circuit board. Complicating the situation is the increasing complexity of electronic components such as integrated circuits which require a high pin count package to electrically connect the device to a user system.
Electronic circuits for complex systems such as digital computers typically are comprised of a multiplicity of interconnected integrated circuit chips. The integrated circuit chips are made from a semiconductor material such as silicon or gallium arsenide, and microscopic circuits are formed on the top surface of the chips by photolithographic techniques. In a conventional form of construction, the integrated circuit chips are mounted in respective ceramic packages, and the ceramic packages are mounted on a printed wiring board or printed circuit board. The majority of the ceramic packages are co-fired ceramic packages that are formed by overlaying multiple layers of ceramic in their green state and then simultaneously firing the layers to form a monolithic body. The ceramic packages have numerous external pins which are mechanically attached by solder or socket to conductor patterns printed on a printed circuit board.
Each ceramic package has a central cavity receiving an integrated circuit chip, and a set of conductors called leads connecting the external pins to the integrated circuit chip. Typically the leads are provided by a thick film conductor pattern deposited on a ceramic layer of the package. The conductor pattern includes a number of bonding areas spaced around the central cavity of the package. When an integrated circuit is received in the central cavity, the bonding areas align with respective bonding pads formed in metallization layers on the surface of the integrated circuit chip. The bonding areas of the package are connected to the bonding pads on the chip by thin flexible segments of bonding wire or metal tape that are bonded by thermocompression or thermosonic bonding to the bonding areas and pads.
With processor speeds moving through 100 Mhz and continually upward, the relatively high dielectric constant of alumina ceramic used in ceramic packages is becoming difficult to deal with in terms of semiconductor packaging. Likewise, the relatively high resistivity of the tungsten conductor of the ceramic substrate causes significant design problems. Plastic integrated circuit packages have evolved as a cost effective replacement for ceramic packages. Modern laminate based molded packages offer electrical, thermal and design performance that matches and often times exceeds that of ceramic packages at a lower cost. Electrically, laminate substrates have a clear advantage over co-fire ceramic with both lower resistance wiring and lower dielectric constant. Essentially, electrical designs can be implemented in less than half the volume (and half the number of layers) as an equivalent ceramic based design.
Laminate based packages offer design flexibility, in terms of electrical design and final package configuration that is unsurpassed by any other packaging technology. The ability to use the substrate as the basis of the package, in virtually any form factor, increases the cost effectiveness of the technology. The ability to fit a laminate substrate into virtually any enclosure allows for any level of environmental protection necessary. Along with high electrical performance and high density wiring capability, laminate based packages have thermal expansion characteristics that match those of printed circuit boards very closely. This thermal expansion match is important for preventing thermally induced stress that can quickly fatigue solder joints and stress both the package and the board. There is a continuing need for a cost-effective, improved method for producing laminate based packages.